Understanding the mechanisms by which HIV is able to persist in the face of ART is necessary in order to identify strategies that may interrupt viral persistence and ultimately lead to virus eradication. In this regard there are two important concepts to consider. One is the presence of a long-lasting reservoir of quiescent infected cells that even though productively infected do not produce virus until activated. This has been designated as the latent reservoir and has been the subject of extensive investigations. The second is composed of productively infected cells that continue to produce virus at very low levels despite ART. This has been designated as the residual active HIV reservoir and is the subject of RFA-AI-12-042. The current proposal is submitted in response to this RFA. The active HIV reservoir, particularly in the blood, has been extensively studied. In contrast, because of the intrinsic difficulties obtaining adequate samples for analysis from infected patients undergoing ART we know very little about the residual active HIV reservoir in tissues. Therefore there is a definitive need for an in vivo model where tissue reservoirs can be studied and where therapeutic interventions aimed at virus eradication can be evaluated. Bone marrow-liver-thymus (BLT) mice developed in our laboratory have been validated for the study of HIV latency and persistence. The functionality and usefulness of BLT mice in the study of the residual active HIV reservoir will become evident when the following aims are completed, wherein the parameters of viral persistence will be established and novel interventions will be tested. The long-term goal of our project is to develop and implement a novel, reproducible and flexible experimental platform in which novel protocols for eradication of the residual active HIV reservoir can be evaluated and directly compared for their efficacy. This will serve to inform which strategies have the best likelihood of providing benefit when implemented in humans. Specific Aim 1) To characterize the residual active reservoir present in the tissues of HIV infected BLT humanized mice undergoing ART. Specific Aim 2) To determine the effect of early treatment on the size and distribution of the active HIV reservoir present in different tissues. Specific Aim 3) To establish a platform for the evaluation of targeted interventions aimed at reducing (or eliminating) the residual active HIV reservoir.